1. Field of the Invention
The present invention relates to an optical pickup system, and more particularly, to a spherical aberration compensation actuator and an optical pickup system capable of compensating a spherical aberration by operating a spherical aberration compensation element provided on an optical path in single-axial direction.
2. Description of the Related Art
Due to a high density trend in an optical recording technology, a laser diode of a more shorter wavelength and an object lens having a more greater numerical aperture (NA) are widely used.
With such a trend, an optical system of a blue laser diode (BD) class is suggested. In the optical system of the BD class, a blue laser beam of a high numerical aperture (e.g., NA=0.85) and a short wavelength (e.g., 405 nm) is used.
FIG. 1 is a schematic view illustrating a construction of a related art BD-class optical pickup system.
As illustrated in FIG. 1, the optical system includes: a blue laser diode 101 for generating a blue laser beam; a beam splitter 102 for reflecting or transmitting the generated beam; a collimator lens 103 for converting the incident beam into a parallel beam and illuminating the parallel beam; an object lens 104 for condensing the incident beams from the collimator lens 103 onto an optical disk 105 and delivering a reflected beam reflected from the optical disk 105 to the collimator lens 103; and an optical detector 106 for detecting the beam reflected by the beam splitter 102 in form of an electrical signal.
In operation, a laser beam generated from the BD 101 passes through the beam splitter 102 and the beam that has passed through the beam splitter 102 is converted into a parallel beam by the collimator lens 103 and provided to the object lens 104.
The object lens 104 condenses the incident beams onto one point on the optical disk 105 to record or play information. At this point, the condensed beam is reflected by the optical disk 105 and the reflected beam passes through the object lens 104 and the collimator lens 103 positioned on a reflection path and is incident to the optical detector 106 by the beam splitter 102. Here, the object lens is mounted on the optical pickup actuator and operated in a tracking and a focusing directions.
The optical detector 106 converts reflected and inputted information into an electrical signal, thereby controlling a tracking servo and a focusing servo.
In the meantime, as a high density trend of the disk progresses, various technical difficulties are emerging. A representative difficulty among them is a spherical aberration generated due to use of a large-diameter lens. The spherical aberration is generated because a focus at which light passing through a principle axis of the object lens is condensed and a focus at which light passing through an outer periphery of the object lens is condensed are varied in their position on the same optical axis.
Therefore, the spherical aberration is greatly influenced by a laser wavelength and a thickness of a cover layer of the optical disk.
In the meantime, since the beams are not exactly condensed onto an optical recording medium in an optical system where the spherical aberration is generated much, power transfer of the laser diode gets inefficient and a signal-to-noise (S/N) ratio is deteriorated when reading data. Further, for high integration and high capacity of data, the BD-class optical system has two layers on its disk. In the above BD-class optical system, the spherical aberration generated due to a deviation of the disk cover layer exceeds an optical aberration tolerance, for a wavelength of a light source used is short, or the spherical aberration is generated due to a deviation of each layer while a dual layer disk is used for increasing a storage density. Particularly, an optical element should be offset on an optical path appropriately for a relevant disk layer in order to compensate a spherical aberration due to a deviation of each disk cover layer generated by recording/playing of the dual layer disk.
One of representative methods used for solving the above problems is to correct a spherical aberration by inserting a liquid crystal plate in a general optical pickup structure. Though such a method has advantages of being able to actively control a spherical aberration in a relatively exact manner, there are disadvantages that big spherical aberrations are generated unless the liquid crystal plate is simultaneously moved when the object lens performs a tracking movement, thus the liquid crystal plate should be installed just in front of the object lens and cooperated using an actuator.
Another spherical aberration correction method requires a single-axis driving servo system which moves an optical element on an optical axis to compensate a spherical aberration as illustrated in FIG. 2.
As illustrated in FIG. 2, in a related art single-axis actuator for compensating a spherical aberration, a beam expander 110 for compensating a spherical aberration includes: a collimator lens 112 mounted on a central portion of a lens holder 111; a motor 113 for operating the lens holder 111; a lead screw 114 for being rotated at one side of the lens holder 111 by the motor 113 so as to operate the lens holder 111; and a shaft 115 for guiding movements of the lens holder 111 at the other side of the lens holder 111.
The beam expender moves the lens holder 111 to an optical-axis direction so as to compensate a spherical aberration due to the collimator lens 112. For that purpose, if the motor 113 is operated in a forward or a backward direction, the lead screw 114 connected with a shaft of the motor is rotated to move the lens holder 111 back and forth and the shaft 115 on the other side of the lens holder guides movements of the lens holder 111. Accordingly, an offset of the collimator lens 112 is adjusted and a spherical aberration is compensated.
However, since the lead screw 114 which is the shaft of the motor 113 is installed on one side of the lens holder 111, force that operates the lens holder 111 may be concentrated on one side. Further, in case a lead screw method is used, a separate motor-screw system should be provided, which results in disadvantages in viewpoint of cost and assembly efficiency.
Further, the beam expender requires a high-precision operation for a spherical aberration compensation. In addition, an angle tilt should be minimized during operation in order to secure operation precision of less than several μm and a tilt margin of an optical element. Still further, since feedback for position information is required in real time in case a separate servo system is provided, a circuit system should be additionally provided.